Piezoelectric ultrasonic transducers have been utilized in in-line pipeline inspection since the mid-1980's. Specifically, the use of piezoelectric transducers for pipeline inspection is disclosed in U.S. Pat. No. 3,810,384, issued May 14, 1974, entitled "Ultrasonic Pipeline Inspection Device". The teachings of this patent are incorporated herein by reference.
In the system described in this patent, transducers are used for making measurements of the remaining wall thickness of in-service pipelines. While improvements have been made to existing inspection systems, the use of piezoelectric devices for this application remains a challenge. Typically, the transducers are mounted in a mechanical holder so that a relatively constant distance is maintained between the face of the transducer and the inside of the pipeline wall. The mechanical mounting arrangement is then moved through the pipeline as a part of another vehicle commonly known in the industry as a "pig". The transducers must operate in the pipeline environment to make measurements in determining the mechanical integrity of the pipeline. Pipelines frequently carry liquids or gases at pressures of 100 psi to 2000 psi. A wide variety of different materials are conveyed by pipelines, including caustic compounds, petroleum products containing acids, liquified petroleum products (LPG), carbon dioxide and dry gaseous materials. Further, the temperature of products conveyed by pipelines can vary widely. Of critical importance to the pipeline operator are the effects of corrosion damage to the pipeline. Consequently, it is common to measure the thickness of the pipe wall around the entire periphery of the pipe and at short intervals along the length of the pipeline. In the event that corrosion has taken a toll on the pipe wall, the measurements made by the ultrasonic transducers will indicate that the wall is thinner in the affected zones than it is where no corrosion is present. Although the inspection system is controlled by sophisticated computer based electronics, the performance of the ultrasonic transducers is critical to acquisition of meaningful inspection data.
Piezoelectric transducers are manufactured by mechanically configuring ceramic material to provide a natural frequency of vibration compatible with the testing necessary for the pipeline. Ordinarily a transducer crystal is produced in the form of a thin circular wafer. When a short duration voltage (pulse) is applied between the front and back surfaces of the wafer, it responds by changing its physical dimensions and consequently, experiences a mechanical vibration. The physical nature of the crystal vibration is controlled by the elastic properties of the piezoelectric material and its geometric dimensions. The frequency of the resulting vibration is called the natural frequency of the crystal. When placed in proximity to another elastic material, it can stimulate the propagation of acoustic energy in that material. Acoustic energy is propagated according to the physical theory governing the transmission of waves in elastic media. The normal process for stimulating elastic waves in a material undergoing nondestructive testing is to couple the transducer to the specimen material via a fluid. Nondestructive testing done in this manner is sometimes called direct contact testing. Direct contact testing is used during external pipe wall testing. If the pipe has been buried it has to be excavated for direct contact testing at considerable expense. Among the common fluids used in standard thickness testing is glycerin and silicon oils. These "couplants" are usually quite viscous with the consistency of a heavy grease. These are unsuitable for the in-line inspection of pipelines because of their viscosity. Therefore, other materials must be utilized for this purpose.
Clearly, during the process of inspecting a pipeline, it is convenient to use the materials that are ordinarily transported by the pipeline. In petroleum related transportation systems, the materials range from heavy crude oils to natural gas. The piping industry desires to inspect all kinds of pipelines regardless of the materials that are being transported. Unfortunately, existing commercial inspection systems utilizing ultrasonic transducers perform satisfactorily only in certain liquid pipelines. The reason for this limitation is based on the nature of piezoelectric transducers. Specifically, difficulty arises from the differences in physical properties of the transducer and the pipeline materials. All physical materials exhibit a property known as acoustic impedance. This property, for practical purposes, is specified by the arithmetic product of the material density and the speed with which sound is propagated in the material. It is well known that the acoustic impedance of piezoelectric ceramics is considerably higher than that of virtually all fluids commonly transported in pipelines. The impedance mismatch results in a variety of difficulties when trying to use monolithic piezoelectric devices as pipeline inspection transducers where many different pipeline fluids are involved. Acoustic impedance is analogous to electrical impedance and it is well known that, between two electrical circuits, maximum power transfer occurs only when the impedance of the driven circuit matches the output impedance of the driver circuit. When the impedance is complex, the input impedance of the receiving circuit must be the complex conjugate of the output impedance of the driver. Also, the mathematical models for acoustic transmission verify that maximum acoustic energy is transmitted between two acoustic media when the two media have identical acoustic impedance's.
A commonly used ceramic piezoelectric material is PZT (Lead Zirconate Titanate) and this material has an acoustic impedance of about 30 MRayl (1 MRayl-10.sup.6 kg/m.sup.2 -sec). Many pipeline liquids have approximately the same acoustic impedance as water which is about 1.5 MRayl. This means that the transducer/water impedance ratio is about 20:1 which is a significant mismatch and results in a large part of the acoustic energy produced by the transducer being reflected back into the transducer. Additional problems include narrow bandwidth and minimal damping. However, in spite of these serious drawbacks, sufficient energy is transmitted to the liquid to make in-line pipeline inspection practical.
The pipeline liquid is called a couplant since it is the means by which the acoustic energy emitted by the transducer is "coupled" to the steel pipe wall. So, while the transducer/liquid match is not ideal, it is close enough to be useful with certain fluids. In a typical pipeline inspection configuration, the transducers are directed normal (perpendicular) to the pipe wall and are positioned such that the transducer face is in the range of 0.5" to 1.5" from the inside pipe surface. It is critical that the transducers be efficient at producing and receiving ultrasonic energy because a considerable part of the acoustic energy never enters the couplant. Also, pipe wall abnormalities will scatter some of the acoustic energy and there is significant signal attenuation in the couplant.
In most practical ultrasonic pipeline inspection systems, transducers are utilized as both a transmitter and a receiver of acoustic energy. This is accomplished by the so-called "front-end" electronics which connects the transducer terminals to a high-power driver amplifier during the transmit interval and then switches the transducer terminals to a high gain, low noise amplifier during the receive interval. Generally, a transducer that exhibits poor characteristics as either a transmitter or a receiver will also be inefficient at the other task.
If the effectiveness of energy transfer between the transducer and the couplant is less than ideal for liquid couplants, it is next to impossible when the couplant is air or gas. The acoustic impedance of air at standard temperature and pressure is approximately 4.times.10.sup.-4 MRayl. This means that the ratio of the acoustic impedance of PZT to that of air, under these conditions, is about 7.5.times.10.sup.4. This extreme difference in acoustic impedance's results in virtually all of the acoustic energy produced by the transducer being reflected back into the transducer. Because no energy is propagated into the couplant, there is no means for acquiring reflected energy from the pipe wall. Thus, the inspection of gas transmission pipelines has not proven to be practical with monolithic piezoelectric transducers.
This application is based on the discovery that specifically configured, multi-element ultrasonic transducers exhibit characteristics of lower density, thus lower acoustic impedance, with increased elastic compliance that make them compatible with conditions associated with the ultrasonic inspection of pipelines. It has been discovered that by effectively arranging a large number of small piezoelectric elements in an array such that all elements are electrically driven from a single source an assembly is provided that exhibits properties analogous to the multi-element arrays used in advanced radar systems.
Multi-element transducers have been evaluated by substituting them in place of the monolithic devices that are ordinarily used in pipeline equipment. Their performance in the pipeline environment was found to be unexpectedly superior to that of conventional monolithic transducers. Further, investigation revealed that improved performance has resulted from the fact that multi-element transducers can be fabricated in such a manner that their acoustic impedance's more nearly match that of the common liquids transported in pipelines. Indeed, this disclosure is based on the discovery that the physical configuration of multi-element transducers is such that the devices can be tailored to provide closer acoustic impedance matches to all pipeline fluids from heavy crude oils to gases. Utilization of multi-element transducers makes it possible to design transducers for every type of pipeline fluid.
Ultrasonic pipeline inspection ordinarily involves the measurement of the thickness of the remaining pipe wall. This means that, if no metal loss has occurred in the pipe wall due to corrosion or other mechanical damage, the instrumentation associated with the ultrasonics system will indicate normal wall thickness. However, if metal loss has occurred, the system will record information that indicates that the pipe wall is now thinner than that of the original, undamaged pipe. Traditionally, the ultrasonic process has been to simply measure the time the ultrasonic energy takes as it enters the pipe wall, reflects from the outer wall and returns to the transducer. For this measurement, the reference is the first reflection from the inside pipe wall (ID) surface. The next signal received from the transducer is ordinarily the reflection from the outside (OD) pipe wall. The time difference from the beginning of the ID signal to the start of the OD signal represents the time taken for the ultrasonic energy to traverse the pipe wall twice. This is commonly called two-way time and in pipeline inspection parlance it is often called "metal time" because it represents the time the ultrasonic energy takes to traverse the steel wall of the pipe. Using half the metal time (one-way time) the pipe wall thickness is readily computed because the velocity of sound in steel (approximately 5,793 m/sec.) is known.
Applicants have determined that this conventional method for measuring pipe wall thickness is generally unsuitable for use in pipeline inspection. For example, this technique is inappropriate for determining if a metal loss defect is internal or external to the pipe. There are many other uncertainties that arise when trying to use the conventional method for wall thickness measurement. Extensive laboratory and field testing has demonstrated that the validity of the wall thickness measurement process is dramatically improved if multiple metal times are utilized in an appropriate computer processing algorithm. Multiple metal times are produced in the ultrasonic wall thickness measurement process because of the significant acoustic impedance contrasts between the steel pipe wall and the couplant on the inside of the pipe and air or pipe coating on the outside of the pipe. The magnitude of each successive metal time pulse is reduced exponentially. The reduction of the signal amplitude is caused by a combination of the interface impedance contrasts and attenuation in both the steel and the couplant. This is readily modeled mathematically as a decaying exponential function and the time constant controls the rate of decay of the signal amplitude. The significance of the metal time amplitude decay is that the signal to noise ratio must be acceptable for the last metal time used in an analysis. For example, if five metal times are used in an analysis algorithm in order to facilitate the final interpretation process, the last metal time must still be well above a threshold that exceeds the background noise in order to be used in the algorithm.
While conventional monolithic piezoelectric transducers have provided usable multiple metal time signals for the analysis algorithm, the results are often less than satisfactory and, for some pipeline fluids, the results have not proven to be satisfactory. The signal is often so small, even after two metal times, that background noise causes difficulty with the analysis algorithm and a meaningful wall thickness measurement cannot be made.
In trying to improve upon the signal to noise ratio inherent in conventional monolithic piezoelectric transducer, applicants have discovered that multi-element transducers significantly improve the signal to noise ratio so that multiple metal times can be used in the analysis algorithm. Therefore, by using multi-element transducers, a significantly larger number of wall thickness measurements can be made. Indeed, multi-element transducers have been found to improve the sensitivity of the transmit/receive process by nearly 10:1 (20 db). Perhaps of even more significance, the decay time (time constant) of the successive metal time pulses has been improved so that many more metal times can be utilized without significant problems with signal to noise ratios.